Checkdown

What is Checkdown?

A term for when a quarterback throws a short pass to
a nearby player after a passing play has had time to develop.
Usually, a checkdown is used when the primary receivers are not
open, and the quarterback wants to get rid of the ball for some
yardage rather than risk a sack. The receiver on a checkdown is
usually a running back that comes out of the backfield or swings to
the side of the field, and will attempt to gain extra yardage after
receiving the pass.

Sporting Charts explains Checkdown

Most pass plays will have a designed checkdown as a
safety valve for the quarterback. The checkdown is there after the
quarterback has gone through his entire progression and not been
able to find an open receiver. Since the pass rush has the
advantage the longer a play goes, a quarterback will often take the
easy pass and gain some yardage rather than get sacked. The most
common checkdown route is having a running back slip out of the
backfield and turn around for a reception just a few yards
downfield.